1. Field of the Invention
The present invention relates to pumps for pumping shear sensitive material, and in particular, but not exclusively, to pumps for pumping viscous fluids which are prone to set if subjected to high frictional forces.
One such viscous fluid is UV ink, which is an ink of a high viscosity which cures when subjected to ultra violet radiation. The properties of the UV ink which enable it to set when it is irradiated unfortunately render it very sensitive to friction. Viscous fluids require the build up of a high pressure in order that they can be pumped.
2. Description of the Prior Art
A pump is known which provides very little frictional force on the material being pumped and that is a double diaphragm pump; such pumps do not have sliding surface between which the material to be pumped travels and therefore there is little chance of a high frictional force being generated which will cure the material. However, due to the inherent limitation of the flexible diaphragm material employed, such pumps are restricted to low pressure applications, that is up to about 13.6 atmospheres (200 psi) Therefore, such pumps are unsuitable for pumping material of a high viscosity.
Single and double acting pumps can each provide the required pressure to pump viscous fluids.
Both double acting pumps and some single acting pumps have a piston which is reciprocated between an inlet and an outlet such that fluid is forced from below the piston to above the piston during a pumping cycle.
In a double acting pump fluid is forced between two separate chambers in the pump, the piston doubles as a valve between the chambers and the fluid is Forced through small apertures which open between the two chambers during displacement of the piston. In some types of single acting pump the piston has several small apertures through which the fluid is forced, during the reciprocating movement of the piston,in order to move fluid from the inlet to the outlet. Each of the above pumps therefore have the drawback that the small clearances and the meshing of several opposing surfaces, can produce a high frictional force on the fluid being pumped which enable shear sensitive materials such as UV ink to cure within the pump and consequently block the pump and prevent it operating.
In a further known type of single acting pump, used for pumping light viscosity fluids, such as hydraulic oil, a displacement rod is reciprocated within a displacement chamber, by which means fluid is drawn in through the inlet into the displacement chamber on one cycle of the rod and from the displacement chamber out through the outlet on the other cycle of the displacement rod. However, if such pumps are used to pump highly viscous fluids, voids form in the displacement chamber, thereby making them unsuitable for pumping such fluids.
It is an object of the present invention to provide a pump which overcomes, or alleviates, the aforementioned problems.